The present invention is directed to low resistance electrical contacts on metal oxide superconductors and a method for making such contacts.
In order to make commercial use of superconductive materials, particularly metal oxide superconductors such as YBa.sub.2 Cu.sub.3 O.sub.z (YBCO) and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.z (Bi(2212)), it is necessary to make electrical connections to other electrically conductive materials, such as wires and other electrical components, including other superconductive materials. It is desirable that such electrical contacts be compatible with known methods for interconnecting electrically conductive materials and components, such as soldering, welding, wirebonding and other interconnection methods. In the past, efforts have been made to form conductive contacts on metal oxide superconductors using various noble metals such as Ag, Au and Pt, however, the methods employed have produced limited success.
One method of forming such contacts has employed physical vapor deposition such as sputtering or evaporation to deposit a metal contact directly on a metal oxide surface. Electrical contacts formed using this method have shown poor solderability in tests conducted by Applicant. It is believed that such solderability problems are related to the thickness of the metal contacts deposited using physical vapor deposition techniques which were typically on the order of 0.1 to 1 microns. Thicker contacts may be obtained using physical vapor deposition techniques, but deposition of thicker noble metal films using these techniques may not be desirable due to the length of the deposition times required to deposit thicker metal contacts, as well as the inefficient use of the noble metal deposition materials, in that a significant portion of the starting material used for deposition is deposited on the walls of the deposition chambers used when employing these techniques. In addition, physical vapor deposition techniques such as sputtering or evaporation typically require masking of the substrate material, in order to limit the deposition of the contact material to the area of interest on the metal oxide material.
Another technique for forming metal contacts on metal oxide superconductors has employed the use of silver paint or paste, such as conductive epoxy materials which utilize conductive metal particles suspended in an organic resinous matrix. Such materials can be employed using several methods. One method involves the use of an appropriate hardener to harden the resinous matrix and thus form an electrical contact. Such contacts are known to have higher electrical resistivity than pure metal contacts due the higher resistivity of the organic matrix. A second method is to apply the metal filled resin (without the hardener) as a silver paint or paste in an area of a metal oxide where an electrical contact is desired, and then pyrolize the resin to remove the organic binder leaving a metal contact, such as a silver contact. Applicants have also observed solderability limitations with painted contacts as described above. Namely, when the painted contacts were thin (e.g. relatively few coats) the contacts exhibited solderability problems, and when the painted contacts were thicker (e.g. a plurality of coats of silver paint) the contacts exhibited a propensity to fall off, particularly when exposed to thermal cycling such as encountered during a soldering process.
Yet another related art technique for producing metal contacts on metal oxide superconductors has involved forming the contact into the body of the metal oxide during its fabrication, such as by casting the metal oxide into a mold containing a preform of the desired electrical contact. See Elschner and Bock, Advanced Materials Vol. 4 No. 3, 1992, pp. 242-244. However, one problem observed using this technique is that after a required annealing step to form the metal oxide superconductor, there was no electrical continuity between the contact and the metal oxide. Subsequent annealing steps can be employed to affect the continuity of the contact and produce a relatively low specific surface resistivity between the contact and the metal oxide superconductor on the order of 2.5 .mu..OMEGA.-cm.sup.2. However, casting is not a desirable method for forming all metal oxide superconductors of interest, and the subsequent anneal adds an additional process step and therefore additional cost to produce these contacts. Also, there is concern about the susceptibility of such contacts to failure by separation of the metal/metal oxide interface due to the thermal expansion mismatch of these materials, especially during subsequent electrical interconnection operations or in a cyclic thermal environment.